Generally, concrete is a brittle material with high compressive strength but low tensile strength. In the concrete industry, all concrete work is typically specified on the basis of the compressive strength. Any attempt to improve the crack strength (tensile strength) and toughness of the concrete almost always requires the introduction of reinforcing addition. For example, rebar (steel rods) are added which provides structural integrity but does not eliminate cracking. Metal mesh has also been added to reduce cracking but it cannot be used effectively to reinforce concrete of complex geometry.
Fine monofilaments of plastic fibers have been used to improve the tensile strength and to reduce plastic shrinkage cracking. Plastic shrinkage cracking occurs from constraints on the shrinkage as it sets. The constraints arise, for example, from the concrete being cast on a bed of rocks to make a road. However, these fine monofilaments almost have no effect on the tensile or bending strength of the cured concrete. To improve the strength (i.e., improve the structural integrity of the concrete), steel rebar or mesh have generally needed to be added.
Examples of plastic fibers include polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), aramids (e.g., KEVLAR) and polyvinyl alcohol fibers. However, all of these fibers suffer from one or more problems, such as high cost, low alkaline resistance, low tenacity or low interfacial bonding between the concrete and the fiber.
Polypropylene and polyethylene have been the most preferred fiber to date due to their high tenacity and low cost. Unfortunately, these fibers suffer from very low interfacial bonding. To remedy this problem, coatings have been formed on the surface of the fibers by applying a liquid, such as gycerol ether or glycol ether on the fiber surface, as described by WO 980766. Coatings have also been applied by vapor deposition, such as described by JP60054950. Similarly, chemically modifying the surface has been done, such as described by JP 10236855 (treatment of the surface of a polyoxyalkylenephenyl ether phosphate and polyoxalkyl fatty acid ester). Unfortunately, these methods naturally lead to increased cost, complexity and potentially insufficient bonding of the coating to the fiber.
Another remedy has been the incorporation of inorganic particles in and on the fiber, such as described by JP 07002554. Unfortunately, the fiber process becomes much more difficult (e.g., fiber breakage) and increases the cost and generally decreases the tenacity of the fiber.
Further, it is known that larger fibers are preferable for improving the toughness of the concrete and strength. Unfortunately, larger fibers further exacerbate the problem of bonding with the concrete matrix because of reduced surface area. In addition, none of these methods address another problem associated with plastic fibers in concrete, which is the tendency of larger fibers to clump together into balls that are difficult to break up when added to concrete, resulting in reduced properties of the concrete.
Accordingly, it would be desirable to provide an improved fiber for improving the properties of concrete, for example, that improves one or more of the problems of the prior art, such as improving the plastic shrinkage cracking and strength without the problems large fibers cause.